In recent years, a porous membrane is utilized in a variety of areas, for example, a water treatment field such as water purification treatment and wastewater treatment, a medical application such as blood purification, a food industry field, a battery separator, a charged membrane, and an electrolyte membrane for fuel cells.
Among others, in the drinking water production field and industrial water production field, i.e., the water treatment field such as usage for water purification treatment, wastewater treatment and seawater desalination, a porous membrane is used as an alternative to conventional sand filtration, coagulating sedimentation, evaporation, etc. or for enhancing the quality of treated water. In these fields, since the amount of treated water is large, a porous membrane with excellent water permeation performance makes it possible to reduce the membrane area, provide a compact apparatus and in turn, save the equipment cost and is advantageous in view of membrane exchange cost or footprint.
As the porous membrane for water treatment, a membrane appropriate to the size of a separation target substance contained in water to be treated is used. Usually, natural water contains many suspended components, and a microfiltration membrane or ultrafiltration membrane for removal of suspended components in water is therefore used in general.
In the water treatment, for the purpose of sterilizing permeate or preventing biofouling of separation membrane, a sterilizer such as sodium hypochlorite may be added to the portion of separation membrane module, or as the chemical cleaning of separation membrane, the separation membrane may be washed with an acid such as hydrochloric acid, citric acid and oxalic acid, an alkali such as aqueous sodium hydroxide solution, chlorine, a surfactant, etc. Accordingly, a separation membrane using, as a material having high chemical resistance, a fluororesin-based polymer typified by polyvinylidene fluoride has been recently developed and utilized.
In the water purification treatment field, a problem of a chlorine-resistant pathogenic microorganism such as cryptosporidium getting mixed in with drinking water has become tangible since late 20th century, and strength high enough to allow for no mixing of raw water by preventing membrane breakage is required for the porous hollow-fiber membrane.
In order to obtain a porous hollow-fiber membrane having high water permeability, high strength/elongation and high chemical resistance, various methods have been heretofore proposed. For example, Patent Document 1 discloses a wet solution method using a fluororesin-based polymer. Specifically, in Patent Document 1, a polymer solution prepared by dissolving a fluororesin-based polymer in a good solvent is extruded through a spinneret at a fairly lower temperature than the melting point of the fluororesin-based polymer to put the polymer solution into contact with a liquid containing a non-solvent for the fluororesin-based polymer to thereby form an asymmetric porous structure by way of non-solvent induced phase separation.
However, in the wet solution method, it is difficult to cause phase separation uniformly in the membrane thickness direction, and since a membrane having an asymmetric three-dimensional network structure containing macrovoids is formed, the strength is disadvantageously insufficient. In addition, there is a drawback that many membrane-forming conditions and factors affect the membrane structure or membrane performance and therefore, not only the membrane-forming process is difficult to control but also the reproducibility is poor.
Patent Document 2 discloses a melt-extraction method. Specifically, the following method is described in Patent Document 2. A fluororesin-based polymer is melt-kneaded with an inorganic fine particle and an organic liquid to obtain a membrane-forming raw liquid. This membrane-forming raw liquid is extruded through a spinneret at a temperature not less than the melting point of the fluororesin-based polymer and cooled/solidified. Thereafter, a porous structure is formed by extracting the organic liquid and inorganic fine particle. In the case of melt-extraction method, the void characteristics are easy to control, and a membrane having a relatively homogeneous three-dimensional network structure is obtained without forming macrovoids. However, the strength thereof is not sufficient, and if the inorganic fine particle exhibits poor dispersibility, a defect such as pinhole may occur. Furthermore, the melt-extraction method has a drawback that the production cost becomes extremely high.
Patent Document 3 also discloses a melt-extraction method. In Patent Document 3, two kinds of fluororesin-based polymers differing in the weight average molecular weight are used and after adding a plasticizer and a good solvent thereto, the resulting mixture is melt-extruded into a hollow-fiber membrane shape, cooled/solidified, subjected to extraction of the plasticizer, and further drawn to obtain a porous hollow-fiber membrane in which a mixture of a crystal oriented portion and a crystal non-oriented portion is observed.
Patent Document 4 discloses a method in which a fluororesin-based polymer solution containing a fluororesin-based polymer and a poor solvent therefor and having a temperature not less than the phase separation temperature is ejected into a cooling bath at a temperature not more than the phase separation temperature and solidified to obtain a hollow-fiber membrane.
Furthermore, in Patent Document 5, a fibrous texture having a diameter of 0.9 μm to 3 μm and being oriented in the length direction of a porous hollow-fiber membrane containing a fluororesin-based polymer accounts for 30% or more of the entire porous hollow-fiber membrane, whereby a porous hollow-fiber membrane excellent in strength and pure-water permeation performance is obtained.